I have yet to play a Dark Knight. Here is how I would imagine one.
In Social situations they are manipulative and very subtle about it. Achieving there own goal is what they to achieve, by any menas necessary. Typically they act somewhat arrogant, but know when be nice, smile or show respect for others. Lying is as natural as breathing and they don't shy away from gaslighting someone for their own benefit.
Sexually they are a mixture of explorative and impulsive in a corrupted way. I imagine them as having alot of sexually motivated intrusive thoughts. But instead of being disturbed, they see them as a opportunity of sorts and imagine the possibilities. For example the corrupted Lupines. A non-corrupted champ has the mindset rape cave = bad. A Dark Knight would imagine. What would if X companion "trips" when running from them. How would the lupines fuck them. Why not watch them from the bushes and enjoy the show? Then rescue your companion, but instead of leaving fuck the comapnion yourself. So that all of the lupies may see that your companions are your bitches and yours alone.
The absolute overindulgence in any sexual act is aslo a key feature. Like when you do Caits side quests and she offers you to take her anal virginity. Non-corrupted champs would be like "A very intimate gesture and I appreciate that you trust me" or "Nice, fat kitty ass". While a Dark Knight would be "How many orgasms does it take, till she vomits my cum?"
The changing description of the Blight Tendril descripes it perfectly. From the wiki: Just touching it gives you a feel of (Dark Knight:excitement |else:dirtiness), a sure sign of its perverse taint.
Dark Knights seek pleasure. They willingly break taboos to experience forbidden pleasures. In a way they are feral and primeval. Pleasure is pleasure. No pleasure is forbidden. They experience pleasure, because they wish to do so.
My view on how a Dark Knight should behave is inspired by Slaanesh from Warhammer. Basically: "Seek new and more intense Sensations. No matter, if positive or negative, or the cost." But in a sexually focused and not grim dark way.